The time between
seeing a lightning flash and hearing the thunder it
produces is a rough guide to how far away the
lightning was. Normally, thunder can be heard up to
10 miles from the lightning that makes it. Lightning
heats the air around it to as much as 60,000
degrees, producing sound waves by the quick
expansion of the heated air. Since light travels at
186,000 miles per second, you see the lightning the
instant it flashes. But sound, including thunder,
travels about a mile in five seconds near the
ground. If 15 seconds elapse between seeing a
lightning bolt and hearing its thunder, the
lightning was about three miles away.
Lightning closer than about three miles away is a
warning to take shelter immediately. Successive
lightning strikes are often two to three miles
apart. If the first stroke is three miles away, the
next one could hit you!